Drivers and Driving Methods for a LED String, Capable of Providing LED Short Protection or Avoiding LED Flickering

ABSTRACT

The disclosure regards to drivers and driving methods for a LED string consisting of LEDs. The LED string and a current switch are coupled in series between a power line and a ground line. The power line is powered to regulate a signal representing a current passing through the LED string. An enable signal capable of switching the current switch is provided. Whether a predetermined event occurs is detected. When the predetermined event occurs, the enable signal is clamped to have a predetermined logic value, the current switch thereby being kept either open or short.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of TaiwanApplication Series Number 101136768 filed on Oct. 5, 2012, which isincorporated by reference in its entirety.

BACKGROUND

The present disclosure relates generally to drivers and driving methodsfor light emitting diodes (LEDs).

Superior in power conversion efficiency, compact product size, and lifespan, LEDs are broadly popular in the industries of house lighting andpanel backlight. For example, a great number of LCD backlight panels arecurrently using LED modules for backlight, rather than CCFL modules thatwere commonly adopted several years ago.

FIG. 1 demonstrates a LED driver 10 capable of being used in a backlightmodule. The LED driver 10 drives a LED string 12 consisting of LEDsconnected in series. In the LED driver 10, a booster 18 converts powersource V_(IN) at a major power line to power source V_(OUT) at an outputpower line OUT. A LED string 12 and a current switch 22 are connected inseries between the output power line OUT and a ground line GND. A powercontroller 14 periodically turns ON and OFF a power switch 16 to controlthe power conversion of the booster 18. While powering the output powerline OUT, the major purpose of the power controller 14 is to stabilize afeedback voltage V_(FB) at the feedback node FB, equivalentlystabilizing the current flowing through the LED string 12 and thebrightness of the LED string 12.

A dimming signal S_(DIM) is fed to the enable node EN of the powercontroller 14. A level shifter 20 shifts the dimming signal S_(DIM) witha logic level of 5V to become a switch signal S_(MOSDIM) with a logiclevel of 12V. When the dimming signal S_(DIM) is 5V in voltage level, or“1” in logic, the current switch 22 is ON, performing a short circuit,and the power controller 14 periodically switches the power switch 16 toregulate the feedback voltage V_(FB). Accordingly, the LED string 12illuminates stably.

When the dimming signal S_(DIM) is 0V in voltage level, or “0” in logic,the current switch is OFF, performing an open circuit, and the powercontroller 14 constantly turns OFF the power switch 16. As there is nopower converted to power the LED string 12, it darkens.

The design of the LED driver 10 shall take several abnormal events, suchas LED open, LED short, output over voltage, or flickering, to name afew, into consideration. For example, if power source V_(OUT) at anoutput power line OUT is over high, it might impose electric shock tocareless operators or fire accident to environment, such that outputover voltage should be prevented.

SUMMARY

Embodiments of the present invention disclose a driver for driving a LEDstring consisting of LEDs. The driver has a current switch and aswitched mode power supply. The current switch is connected in serieswith the LED string between a power line and a ground line. The switchedmode power supply powers the power line to regulate a signalrepresenting a current passing through the LED string. The switched modepower supply comprises an enable node and a clamping circuit. An enablesignal at the enable node is capable of switching the current switch.When a predetermined event occurs the clamping circuit clamps the enablesignal to have a predetermined logic value, the current switch therebybeing kept either open or short.

Embodiments of the present invention disclose a driving method for a LEDstring consisting of LEDs. The LED string and a current switch arecoupled in series between a power line and a ground line. The power lineis powered to regulate a signal representing a current passing throughthe LED string. An enable signal capable of switching the current switchis provided. Whether a predetermined event occurs is detected. When thepredetermined event occurs, the enable signal is clamped to have apredetermined logic value, the current switch thereby being kept eitheropen or short.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by the subsequent detaileddescription and examples with references made to the accompanyingdrawings, wherein:

FIG. 1 demonstrates a LED driver in the prior art;

FIG. 2 shows a system circuit to mimic the event when the LED driver ofFIG. 1 encounters an LED short event;

FIG. 3 shows an LED driver to drive a LED string according toembodiments of the invention;

FIG. 4 illustrates time diagrams for signals in FIG. 3;

FIG. 5 demonstrates another LED driver according to embodiments of theinvention;

FIG. 6A and FIG. 6B shows two time diagrams; and

FIG. 7 demonstrates a power controller according to embodiments of theinvention.

DETAILED DESCRIPTION

FIG. 2 shows a system circuit to mimic the event when the LED driver 10of FIG. 1 encounters an LED short event. In comparison with the LEDdriver 10 of FIG. 1, additionally in FIG. 2 is a switch 24, connected inparallel to the LED string 12. As mentioned in the section of“background”, when the dimming signal S_(DIM) is “1” in logic, thecurrent switch 22 is ON and the power controller 14 periodicallyswitches the power switch 16. In the meantime, if switch 24 suddenlyswitches to perform a short circuit, which imitates the happening of anLED short event when all the LEDs in the LED string 12 are all shorted,the feedback voltage V_(FB) rises quickly as being pulled by the powersource V_(OUT), which could be as high as 100V. In case there is nostrategy designed to encounter the LED short event, the power controller14 suffers the over high feedback voltage V_(FB), and risks itself inovervoltage damages or fire accidents caused. Analogously, as feedbackvoltage V_(FB) rises, a large amount of current will pass through thecurrent switch 22 and resister 26, probably causing overheat damage orgetting fire.

FIG. 3 shows an LED driver 30 to drive the LED string 12 according toembodiments of the invention. Designed in the LED driver 30 has LEDshort protection, which prevents damages or risks caused by an LED shortevent from happening. The LED driver 30 has a switched mode power supply32, a dimming controller 34 and a current switch 22.

The dimming controller 34 has a resister 38 and a level shifter 20. Theresistor 38 is coupled between a dimming node DIM and an enable node ENof the power controller 36, and the level shifter 20 between the enablenode EN and a control node of the current switch 22. In the occasionswhen the power controller 36 does not drive the enable node EN, thepower controller 36 provides high input impedance to the enable node EN,and a dimming signal S_(DIM) at the dimming node DIM alone controls thepower controller 36 and the current switch 22. In other words, in theseoccasions, an asserted dimming signal S_(DIM), “1” in logic, enables thepower controller 36 to cause power conversion for powering the outputpower line OUT, and turns ON the current switch 22, while an desserteddimming signal S_(DIM) disable the power controller 36 to interrupt thepower conversion and turns OFF the current switch 22.

Nevertheless, in some occasions the power controller 36 does drive theenable node EN, clamping the enable signal S_(EN) to be either “1” or“0” in logic, based on different conditions. When the enable node EN isdriven by the power controller 36, due to the existence of the resistor38, the enable signal S_(EN) has, for controlling the current switch 22,a higher priority than the dimming signal S_(DIM). In other words, whenthe power controller 36 drives the enable node EN, the logic levels ofthe enable signal S_(EN) and the dimming signal S_(DIM) might differ,and the current switch 22 is under the control of the power controller36. When the power controller 36 leaves the enable node to be high inputimpedance, the current switch 22 is under the control of the dimmingsignal S_(DIM), and both the enable signal S_(EN) and the dimming signalS_(DIM) share the same logic value.

In one embodiment, when the enable signal S_(EN) is asserted, “1” inlogic, the power controller 36 generates pulse-width-modulation (PWM)signal S_(DRV) at the driving node DRV, to periodically switch the powerswitch 16 for powering the output power line OUT and building up powersource V_(OUT), to regulate the feedback voltage V_(FB). For example,the power controller 36 modulates the duty cycle of the PWM signalS_(DRV) to stabilize the feedback voltage V_(FB) at 0.3V, and toaccordingly provide a substantially constant current for lighting theLED string 12. At the same time, the asserted enable signal S_(EN), viathe level-shifting provided by the level shifter 20, turns ON thecurrent switch 22.

When the enable signal S_(EN) is deasserted, “0” in logic, the powercontroller 36 makes pulse-width-modulation (PWM) signal S_(DRV) “0” inlogic, to constantly turn the power switch 16 OFF, stopping the powerconversion. At the same time, the deasserted enable signal S_(EN), viathe level-shifting provided by the level shifter 20, becomes the signalS_(MOSDIM) to turn OFF the current switch 22.

Included in the power controller 36 are a booster controller 39, acomparator 40, a SR flip-flop 42, a clamping switch 44 and a timer 46,where the latter five apparatuses construct a LED protection circuit.

The comparator 40 detects the feedback voltage V_(FB), which equallyrepresents the current flowing through the resistor 26 and the LEDstring 12. During normal operation when no abnormal event happens andthe LED string 12 is intended to illuminate, the feedback voltage V_(FB)is about 0.3V, less than 0.5V, such that, in logic, the output of thecomparator 40 is “0”, the Q output of the SR flip-flop 42 “0”, causingthe clamping switch 44 an open circuit. As the clamping switch 44 doesnot clamp the enable node EN to the ground line, the enable signalS_(EN) could have the same logic value with the dimming signal S_(DIM).

As discussed previously, once an LED short event occurs to the LEDstring 12, the feedback voltage V_(FB) rises from 0.3V, quickly. At thetime when it exceeds 0.5V, the comparator 40 sets the SR flip-flop 42and the fault signal S_(Fault) output from the SR flip-flop 42 becomes“1” in logic, turning ON the clamping switch 44 and clamping the enablenode EN to the ground line. The enable signal S_(EN) becomes solidly“0”, irrespective of which logic value the dimming signal S_(DIM) is. Asthe enable signal S_(EN) is “0”, the booster controller 39 turns OFF thepower switch 16, and the level shifter 20 OFF the current switch 22.Since the current switch 22 begins reducing the current flowing throughthe resistor 26, the feedback voltage V_(FB) then drops, such that anover-high-voltage feedback voltage V_(FB) is avoided and damagetherefrom is prevented. Accordingly, an LED short protection isperformed.

This LED short protection is not dismissed even when the feedbackvoltage V_(FB) drops down to 0.5V due to the turning OFF of the currentswitch 22. It is because the fault signal S_(Fault) could still be “1”,as memorized by the SR flip-flop 42, to continuously clamp the enablesignal S_(EN) to “0” in logic.

The power controller 36 periodically dismisses the LED short protection,though, to stop clamping the enable signal S_(EN). The timer 46 startstiming at the moment when the enable signal S_(EN) turns to “0” inlogic, or when the falling edge of the enable signal S_(EN) occurs. Oncethe timer 46 finds that the disable time when the enable signal S_(EN)continuously stays in “0” exceeds a preset valid period T_(OUT), itturns the power-saving signal S_(PD) of its output from “0” to “1”, toreset the SR flip-flop 42, to make the fault signal S_(Fault) “0” inlogic, and to release the enable signal S_(EN) from being clamped by theclamping switch 44. After then, the enable signal S_(EN) starts tofollow the dimming signal S_(DIM), and the LED short protection isdismissed. In case that the dimming signal S_(DIM) is “1” in logic andthe LED short event has not been resolved, once the enable signal S_(EN)is seemingly determined to be “1”, the timer 46 resets, the power-savingsignal S_(PD) turns from “1” to “0”, and the feedback voltage V_(FB)starts rising up quickly due to the continuous existence of the LEDshort event. Once again the feedback voltage V_(FB) will exceed 0.5V totrigger the LED short protection. Therefore, the power controller 36periodically activates and dismisses the LED short protection. Only ifthe LED short event is resolved and the timer 46 has reset the SRflip-flop 42 to dismiss the LED short protection, then the powercontroller 36 could be controlled by the dimming signal S_(DIM),operating in a normal condition.

FIG. 4 illustrates time diagrams for signals in FIG. 3, including, fromtop to bottom, the dimming signal S_(DIM), the enable signal S_(EN), thefeedback voltage V_(FB), a signal S_(LEDSHT) that controls switch 24which could mimic the happening of an LED short event, the fault signalS_(Fault), the switch signal S_(MOSDIM) at the control node of thecurrent switch 22, the PWM signal S_(DRV) at the driving node DRV, andthe power-saving signal S_(PD).

Please refer to both FIG. 3 and FIG. 4. At time t₀ when the dimmingsignal S_(DIM) turns from “0” to “1”, the PWM signal S_(DRV)periodically switches, with a cycle time, the power switch 16 ON or OFF,the switch signal S_(MOSDIM) turns ON the current switch 22, and thefeedback voltage V_(FB) starts approaching to 0.3V.

At time t₁, the signal S_(LEDSHT) turns from “0” to “1”, to mimic thehappening of the LED short event. As a result, the feedback voltageV_(FB) rises abruptly from 0.3V.

At time t₂ when the feedback voltage V_(FB) reaches 0.5V, the LED shortprotection is triggered. As analyzed previously, the SR flip-flop 42turns the fault signal S_(Fault) from “0” to “1”, clamping the enablesignal S_(EN) at “0” in logic, such that both the PWM signal S_(DRV) andthe switch signal S_(MOSDIM) both become “0” in logic. Meanwhile, as thefalling edge of the enable signal S_(EN) occurs, the timer 46 startstiming. Furthermore, as the current switch 22 becomes an open circuit,the feedback voltage V_(FB) begins dropping.

At time t₃, the timer 46 acknowledges, by way of its timing, that thedisable time period when the enable signal S_(EN) is “0” has exceeded apreset valid period T_(OUT). In other words, the LED short protectionexpires. Therefore, the timer 46 asserts the power-saving signal S_(PD)to provide a timeout for the LED short protection. The Assertedpower-saving sign S_(PD) resets the fault signal S_(Fault), releasingthe clamping to the enable signal S_(EN) and dismissing the LED shortprotection. It is therefore the enable signal S_(EN) starts to followthe dimming signal S_(DIM). Since the LED short event has not beenresolved as the signal S_(LEDSHT) is still “1”, the feedback voltageV_(FB) rises steeply.

At time t₄, the feedback voltage V_(FB) reaches 0.5V, similar with whathappened at time t₂. Therefore, the LED short protection is triggeredonce more. It can be concluded that if the LED short event is notresolved the power controller 36 will periodically activate and dismissthe LED short protection.

At time t₅ when the LED short event is resolved by means of turning thesignal S_(LEDSHT) to “0”, the LED short protection is not immediatelydismissed because the LED short protection has not expired. The timer 46finds the expiration of the LED short protection at time t₆, such thatthe LED protection is dismissed and the power controller 36 recovers tothe normal operations as it did prior to time t₁.

As shown in FIG. 3, the power controller 36 might have an oscillator 37to decide the cycle time of the PWM signal S_(DRV) and the preset validperiod T_(OUT). In one embodiment, the oscillator 37 decides that onecycle time is 33 micro-seconds, while the present valid period T_(OUT)is 10000 cycle times.

When the enable signal S_(EN) is “1”, asserted, the booster controller39 provides the PWM signal S_(DRV) to periodically switch, with a cycletime defined by the oscillator 37, the power switch 16 ON and OFF, basedon the feedback voltage V_(FB), as illustrated in the period betweentimes t₀ and t₁ of FIG. 4. When the enable signal S_(EN) is “0”,deasserted, the booster controller 39 keeps the PWM signal S_(DRV) asbeing “0”, turning OFF the power switch 16, as illustrated in the periodbetween times t₂ and t₃. In one case that the enable signal S_(EN) staysas being “0” for a very long time exceeding the preset valid timeT_(OUT,) it probably means that the dimming signal S_(DIM) is intendedto be a constant “0” and to continuously darken the LED string 12. Anypower consumed when the LED string 12 constantly darkens is a waste,though. In one embodiment, the asserted power-saving signal S_(PD) setsthe power controller 36 to operate in a power-saving mode, shutting downsome circuits therein to reduce power consumption. For example, in thepower-saving mode, the booster controller 39, the oscillator 37, ortheir combination are shut down to save power. Both the two time periodsfor deciding the entrance of the power saving mode and the expiration ofthe LED short protection respectively are the preset valid time T_(OUT)in length. This invention is not limited to, however. In anotherembodiment, the former is longer than the latter.

Based on the previous teaching, the LED driver 30 of FIG. 3 according toembodiments of the invention can beneficially obtain the followingachievements.

1. LED short protection: When the LED short event occurs the powercontroller 36 could timely switch off the power conversion, preventingthe feedback voltage V_(FB) from being over high to cause any damage.

2. Automatic recovery to normal operation after the LED short eventvanishes: When the LED short protection expires the power controller 36temporarily dismisses the LED short protection, and if the LED shortevent is not resolved this LED short protection resumes soon. A timeoutof the LED short protection is thus provided. During the timeout, if theLED short event vanished, the power controller 36 automatically startsto operate normally, making the LED string 12 illuminate for example.Namely, the timeout of the LED short protection provides an opportunityfor the power controller 36 to automatically recover to its normaloperation if the LED short event is resolved.

3. Power saving: The dimming signal S_(DIM), if having been deassertedfor a long time enough, can render the power controller 36 to operate ina power-saving mode and reduce power consumption.

The prior art taught in FIG. 1 has an unrevealed problem: flickering. Inview of the stabilization of the overall system and the reduction ofswitching loss, the bandwidth of the system response for the LED driver10 cannot be very broad. Generally, the bandwidth of a system designlocates at somewhere between 100 KHz to 300 KHz. This choice of thebandwidth also defines a minimum response delay time, which is how soonthe system responds to a change of an input signal, such as the dimmingsignal S_(DIM), and the broader the bandwidth the shorter the minimumresponse delay time. In case that the Dim-ON time, the pulse width whenthe dimming signal S_(DIM) is “1”, is shorter than the minimum responsedelay time, the LED driver 10 cannot response quick enough to stabilizethe current passing the LED string 12. This unstable current could causethe LED string 12 to illuminate for a while and darken for anotherwhile, and the periodic switching between illuminating and darkening, ifperceivable to humans' eyes, is called as flickering, which is commonlyunwelcome or forbidden for a lighting system.

FIG. 5 demonstrates another LED driver 80 according to embodiments ofthe invention, for driving the LED string 12, where the power controller83 in the switched mode power supply 82 defines a minimum ON time to theLED string 12 to avoid the flickering. The minimum ON time is theminimum time that the LED string 12, if once driven to illuminate, mustlast to illuminate.

The power controller 83 includes a clamping circuit 84, which comprisesa rising-edge-triggered pulse generator 86 and a clamping switch 88.When a rising edge of the enable signal S_(EN) occurs, meaning that itturns from “0” to “1” in logic, the rising-edge-triggered pulsegenerator 86 generates a pulse S_(PLS) with a pulse width of minimumduration T_(MIN-ON). This pulse S_(PLS) makes the enable signal S_(EN)clamped to be 5V in voltage level, or “1” in logic, for the minimumduration T_(MIN-ON), irrespective of the present logic value of thedimming signal S_(DIM). The pulse S_(PLS) vanishes after the minimumduration T_(MIN-ON), and the clamping switch 88 stops clamping theenable signal S_(EN), letting the enable signal S_(EN) follow thedimming signal S_(DIM). As indicated in the previous teaching, when theenable signal S_(EN) is asserted, “1” in logic, the LED string 12illuminates.

Two time diagrams are shown in FIG. 6A and FIG. 6B, respectively, andthe signals in each figure, from top to bottom, are the dimming signalS_(DIM), the pulse S_(PLS), and the enable signal S_(EN). In FIG. 6A,even though the Dim-ON time of the dimming signal S_(DIM) is shorterthan the minimum duration T_(MIN-ON), the duration when the enablesignal S_(EN) is “1” is about the minimum duration T_(MIN-ON) defined bythe pulse S_(PLS), because of the clamping provided by the clampingcircuit 84. In FIG. 6B, the duration when the enable signal S_(EN) is“1” is about the same with the Dim-ON time of the dimming signalS_(DIM), which is longer than the minimum duration T_(MIN-ON). It can beconcluded from FIGS. 5, 6A and 6B that the illumination of the LEDstring 12, once starting, lasts at least the minimum durationT_(MIN-ON), which accordingly defines the minimum ON time of the LEDstring 12. If the minimum duration T_(MIN-ON) is chosen to be longerthan the minimum response delay time of the LED driver 10, theflickering that occurs in the prior art could be avoided.

In one embodiment, the minimum duration T_(MIN-ON) is two or three cycletimes of the PWM signal S_(DRV). In other words, the minimum durationT_(MIN-ON) could be decided by the oscillator 37.

The LED short protection of FIG. 3 and the minimum duration T_(MIN-ON)of FIG. 5 could together be implemented in a power controller, asexemplified in FIG. 7. In an embodiment, the power controller 92 of FIG.7 replaces the power controller 36 in FIG. 3. Inside the powercontroller 92 are the clamping circuit 84 and LED short protectioncircuit 93, both of which are explained and detailed in previousparagraphs with references to FIG. 3 and FIG. 5, such that theirexplanations are omitted for brevity. In FIG. 7, the oscillator 37defines one cycle time of the PWM signal S_(DRV), which could associatewith the preset valid period T_(OUT) and the minimum durationT_(MIN-ON).

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

What is claimed is:
 1. A driver for driving a LED string consisting ofLEDs, the driver comprising: a current switch connected in series withthe LED string between a power line and a ground line; and a switchedmode power supply, for powering the power line to regulate a signalrepresenting a current passing through the LED string, wherein theswitched mode power supply comprises an enable node and a clampingcircuit; wherein an enable signal at the enable node is capable ofswitching the current switch; and when a predetermined event occurs theclamping circuit clamps the enable signal to have a predetermined logicvalue, the current switch thereby being kept either open or short. 2.The driver as claimed in claim 1, wherein the predetermined event is thesignal exceeding a predetermined value, and when the predetermined eventoccurs the clamping circuit clamps the enable signal to be deassertedand to make the current switch open, thereby providing LED shortprotection.
 3. The driver as claimed in claim 2, wherein the switchedmode power supply periodically releases the enable signal from beingclamped.
 4. The driver as claimed in claim 2, wherein the switched modepower supply further comprises a timer for timing a disable time periodwhen the enable signal is deasserted.
 5. The driver as claimed in claim4, wherein, when the disable time period exceeds a preset valid period,the timer provides a power-saving signal to release the enable signalfrom being clamped.
 6. The driver as claimed in claim 5, wherein thepower-saving signal sets the switched mode power supply to operate in apower-saving mode.
 7. The driver as claimed in claim 5, wherein theswitched mode power supply further comprises an oscillator fordetermining a cycle time, and the preset valid period is a plurality ofthe cycle times.
 8. The driver as claimed in claim 1, wherein thepredetermined event is the occurrence of a rising edge of the enablesignal, and when the predetermined event occurs the clamping circuitclamps the enable signal to be asserted and to make the current switchshort.
 9. The driver as claimed in claim 8, wherein a minimum durationafter the happening of the predetermined event the clamping circuitkeeps the enable signal as being asserted, the LED string thereby havinga minimum ON time.
 10. The driver as claimed in claim 1, wherein whenthe enable signal is deasserted the switched mode power supply stopspowering the power line; and when the able signal is asserted theswitched mode power supply powers the power line.
 11. The driver asclaimed in claim 10, wherein if a disable time period when the ablesignal stays as being deasserted exceeds a predetermined period of time,the switched mode power supply begins operating in a power-saving mode.12. The driver as claimed in claim 1, wherein the switched mode powersupply comprises a booster.
 13. The driver as claimed in claim 1,further comprising: a resister, coupled between a dimming node and theenable node; and a level shifter, coupled between the current switch andthe enable node, for level shifting the enable signal to control thecurrent switch; wherein the dimming node is for receiving a dimmingsignal.
 14. A driving method for a LED string consisting of LEDs,comprising: coupling the LED string and a current switch in seriesbetween a power line and a ground line; powering the power line toregulate a signal representing a current passing through the LED string;providing an enable signal capable of switching the current switch;detecting whether a predetermined event occurs; and clamping, when thepredetermined event occurs, the enable signal to have a predeterminedlogic value, the current switch thereby being kept either open or short.15. The driving method as claimed in claim 14, the predetermined eventis the signal exceeding a predetermined value, and the driving methodfurther comprises clamping, when the predetermined event occurs, theenable signal to be deasserted and to make the current switch open,thereby providing LED short protection.
 16. The driving method asclaimed in claim 15, further comprising: timing a disable time periodwhen the enable signal is deasserted; and providing, when the disabletime period exceeds a preset valid period, a power saving signal torelease the enable signal from being clamped.
 17. The driving method asclaimed in claim 16, further comprising: operating, when the disabletime period exceeds a predetermined period of time, a switched modepower supply in a power-saving mode.
 18. The driving method as claimedin claim 17, wherein the preset valid period is the same with thepredetermined period of time.
 19. The driving method as claimed in claim16, further comprising: providing a switched mode power supply operatingwith a cycle time to power the power line; wherein the preset validperiod is a plurality of cycle times.
 20. The driving method as claimedin claim 14, further comprising: providing a dimming signal to controlthe current switch; and making the enable signal having a higherpriority in controlling the current switch than the dimming signal. 21.The driving method as claimed in claim 20, further comprising: couplinga resister between a dimming node and an enable node, wherein thedimming signal and the enable signal are at the dimming and enable nodesrespectively; and level shifting the enable signal to control thecurrent switch.
 22. The driving method as claimed in claim 14, whereinthe predetermined event is the occurrence of a rising edge of the enablesignal, and the driving method further comprises clamping, when thepredetermined event occurs, the enable signal to be asserted and to makethe current switch short.
 23. The driving method as claimed in claim 22,comprising: clamping, during a minimum duration after the happening ofthe predetermined event, the enable signal to be asserted, the LEDstring thereby having a minimum ON time; and stopping clamping theenable signal after the expiration of the minimum duration.
 24. Thedriving method as claimed in claim 23, comprising: providing a switchedmode power supply operating with a cycle time to power the power line;wherein the minimum duration is a plurality of cycle times.